[mrsuciu]

Force a fresh queue check after transfer so the first Publish returns saved data instead of an empty keep-alive. Reset the subscription’s expiration counter after transfer.

Add tests for queued data delivery, the timing bug, and an empty keep-alive after all queued values have been delivered.

Description

A durable subscription could return an empty keep-alive on the first Publish after transfer, even when values had accumulated while the client was disconnected. The server could reuse its previous keep-alive state before checking the queues.
This change forces a fresh queue check after transfer and resets the subscription's expiration counter.
Queued values are delivered first; normal keep-alives resume once the queues are drained.
Tests cover the timing bug, data delivery after reconnecting with SendInitialValues=false, and a keep-alive after draining all queued writes, including across multiple Publish responses.

Checklist

Put an x in the boxes that apply. You can complete these step by step after opening the PR.

• I have signed the CLA and read the CONTRIBUTING doc.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added all necessary documentation.
• I have verified that my changes do not introduce (new) build or analyzer warnings.
• I ran all tests locally using the UA.slnx solution against at least .net framework and .net 10, and all passed.
• I fixed all failing and flaky tests in the CI pipelines and all CodeQL warnings.
• I have addressed all PR feedback received.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.08%. Comparing base (594eb7a) to head (b8da994).

Additional details and impacted files

@@              Coverage Diff              @@
##           master378    #4622      +/-   ##
=============================================
+ Coverage      62.06%   62.08%   +0.01%     
=============================================
  Files            382      382              
  Lines          79514    79518       +4     
  Branches       13927    13927              
=============================================
+ Hits           49349    49366      +17     
+ Misses         25729    25726       -3     
+ Partials        4436     4426      -10     

Files with missing lines	Coverage Δ
...braries/Opc.Ua.Server/Subscription/Subscription.cs	74.04% <100.00%> (+0.08%)	⬆️

... and 15 files with indirect coverage changes

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